I have the PL300 and the 400, running in mono block with Tannoy Kensington and Avant grade DUO XD; if I were to do it again I would buy the 300.
The 400’s offer so little more than the 300s, at a huge expense if you change tubes.
I listen to your speakers with a Diablo 300, just magic combination; I also listened them with a Pathos Heritage, great combo but not as good as with the Diablo 300, the Heritage had a little trouble driving them at low volumes so they did not sound big and relaxed. I auditioned your speakers vs the Tannoy at Upscale with a 300 and 400, the PLs just do not have the drive these speakers need. For the PLs you need 90+ efficiency especially if you listen at normal levels then you can get the best out of the amps. to my ears the 300 drove the Focal better than the 400, but that is my ears, which did not know if it was a 400 or a 300.
If you want to add some tube color get a tube preamp, your speakers are not easy to drive, especially at lower volumes, they need the power to open up and then they are very nice speakers.
Pardon my English, not native :)